The present invention relates to a method and apparatus for controlling pulp refiners and more particularly to refiners used in manufacturing paper.
U.S. Pat. No. 3,568,939 in the name of Brewster et al discloses in detail the existing state of the art in controlling pulp refiners. In general, the major refining action is accomplished by a group of base refiners arranged in various series and parallel flow paths; a tickler refiner provides for fine or high frequency control of the quality of the output stock. One such quality in the case of paper is known as freeness.
An inherent problem in the control of refiners is the time delay between the base refiners and the output of the tickler refiner. such delay is caused in part by the time constants in the refining operations of all of the refiners. In addition, transport delay or lag is caused by a machine chest between the base refiners and tickler refiner which serves as a mixing tank for the output of the base refiners. Typical values are a time constant of five minutes and a transport lag of 3.5 minutes.
As discussed in the above Brewster patent, if there is a deviation of overall refining action from a set point, one would ideally desire that the maximum corrective action initially occur at the tickler refiner. Then, as the effect of subsequent control action at the base refiners is sensed, the loading of the tickler refiner is reduced back to 50% of capacity. In order to accomplish the foregoing, Brewster discloses an improved control system for adjusting the loading on the base refiners as a group.
However, the prior art does not disclose an integrated control system where both the tickler and base refiners are automatically controlled such that the output of the tickler refiner is closely maintained at the desired target with only minor variations. In addition, where the base refiners are arranged in a complex series-parallel matrix no simple control method has been provided. Such a method is, of course, very important where an overall integrated control scheme is desired.